Varied technologies exist to measure fluid levels in containers. These technologies include but are not limited to mechanical sensors, electromechanical sensors, radar sensors, visual sensors, weight sensors, laser sensors, ultrasonic sensors, and capacitance sensors. Fluid characteristics such temperature, viscosity, conductivity, chemical abrasiveness, acidity, and the like may vary during the measurement of sensors from one level to a second level. These variations may offset measures from a sensor relying on a fixed characteristic to determine a precise level in a container. For example, if a mechanical sensor determines the level of a fluid by first measuring the weight in a known container geometry and associated the first weight on a fluid level based on calculations of the volumetric density of the fluid, once the fluid temperature increases, the volumetric density may decrease, raising the level above the calculated value. For this reason, analogous measures performed over a long period of time require recalibration to actual measured levels.
Fluids such as water are known to serve as proper electrical conductors. If a body of water placed between insulated plates is energized at a certain voltage (V) under the strain from the resulting dielectric force field, a conductive fluid is charged (Q). The capacitance (C) of a fluid is a measure of the amount of electricity stored in a fluid volume divided by the potential of the body. The general formula for the determination of capacitance is C=Q/V. The determination of a capacitance (C) when applied to known geometries can be shown to respond to the following equation: C=kA/d, where k is the dielectric constant of the fluid between plates, A is the cross-sectional area of the plates, and d is the distance between the plates. It is understood by one of ordinary skill in the art that correction factors must be applied to the calculation of any capacitance with plates and surfaces of irregular geometries.
Capacitance sensors consist of either placing two polarized bodies at a fixed voltage (V), often insulated in a conductive fluid, or placing a single insulated body within another body and using the general conductive container of the fluid as a pole of the dielectric force field. As the water level rises in the container, not only does the available capacitive volume increase, the contact surface of the fluid with the polarized bodies increase accordingly.
Capacitance sensors are used in a wide range of environments, including at extreme temperature or in toxic environment, since they require no moving parts and are resistant to vibration, even absent a gravitational field. For example, cryogenic fuel levels on spacecraft are measured by capacitance sensors. Capacitance sensors are inherently vulnerable to changes in fluid characteristics since the dielectric constant of fluids may vary greatly with temperature, chemical composition, pollutants, segregation, phase changes, and other fluid characteristics. For example, the presence of salt or the formation of blocks of ice in a body of water can dramatically affect its measure of capacitance and ultimately the fluid level determined by a capacitance sensor. Detection based on capacitance is also limited by nonintrusive size sensors with limited surface area and the need to measure at low voltage levels. Capacitance sensors often operate at minimal detection levels and require redundant measures in order to determine a level within a limited margin of error.
Therefore, there is a need in the art for a capacitance sensor able to self-calibrate along its analog range of measurement at certain fixed fluid levels in order to limit the uncertainties associated with inherent limitations of this type of sensor.